The present invention relates broadly to voice transmission systems and in particular to an automatic authentication apparatus for a voice transmission system.
It has been shown by the experience gained in Viet Nam and earlier that intrusions into voice communications circuits by unfriendly parties was largely successful because of the inability or unwillingness of our troops to use authentication procedures to identify the caller on those voice circuits.
The process of authentication is used to assure a receiver station that the calling party is providing legitimate information or orders. If two stations, A and B are involved in a communication net, the sequence of events is as follows: Station A calls Station B to transmit an order. Station B cannot immediately identify the caller or the calling station A by any means that is independent of the content of the communication. Therefore, station B requests that station A provide a prearranged response to a challenge that he gives. For example, station A calls station B. Station B, unsure of the identity of the caller requests "Authenticate Delta Foxtrot" knowing that a particular response should be made. Station A responds with "I authenticate Golf Hotel" which is the correct response, and is recognized by station B, and the communication is then started. If any other response is given, station B refuses to conduct communications.
The present invention solves the problem of intrusion into military communications by enemy forces. The bogus use of call signs of our stations by enemy troops is well known. Only another identification means will improve the credibility of the station identification. This intrusion can be prevented by the conventionally performed authentication procedures when used. However, battlefield experience has shown that the conventional authentication procedures now used require the radio operator (communicator) to carry paper lists of challenge/response letters or words. If lost or captured, these lists may be used by enemy operators to intrude on the communication. In order that the physical size of the documents be small enough to carry, the contents would have to be kept few in number. To circumvent the need for large lists, the use of the code is varied periodically by changing a "key". With the conventional authentication procedures, the communicator must: a, have the lists readily available; b, remember the key for each use of the list; and c, be able to search through the list to find the proper challenge/response combination. In the dark this would require available light, which in itself could be fatal.
Since the unit described herein would be fully self-contained and semiautomatic in its use; no paper lists would be required, and operation would be performed by the press of a button on the communication set. The key could also be changed rapidly, even during the course of a conversation, and thus prevent the use of recorded intrusion.
This invention is one in which a small unit is attached to each of the transceivers in use. Only two connections are made to the transceiver, an input in parallel with the microphone circuit and an output from the audio output circuit. Instead of the operators being required to carry and read prepared authentication books or sheets, the unit would generate a set of audio tones in digital form which would be transmitted, received and decoded. The responding station unit would generate the "37 correct" response set of tones which are transmitted to the first station. The first station unit then decodes the received response and compares it with the expected response. If the response matches the expected response then an indication is given that the calling station is truly friendly.
The method of generating these codes and their appropriate responses is proposed to be changeable by means of manually entered "keys" which may be easily changed periodically to prevent intrusion by enemies who may have "broken" the key and might have captured an authentication unit. Therefore, it may be seen that the present invention encompasses the concept of a semiautomatic authentication unit which could be attached to voice transceivers to provide an easily used, reasonably rapid and intrusion-resistant authentication capability.